As a radar device of the aforementioned kind, there is conventionally known a device as shown, for example, in Japanese Patent Application No. Hei 7-271514 (filed Oct. 19, 1995, published as Japanese Patent Preliminary Publication No. Hei 8-278362), which obtains, utilizing the spread spectrum modulation technique, correlation between a received signal from an object and a pseudonoise signal (hereunder referred to as "PN code") consisting of a sequence of pseudonoise codes, and detects an object and measures a relative speed thereof and a distance thereto, utilizing a peak value of that correlation. However, measurement of an azimuth of an object is not referred to in this kind of radar device.
As a radar device which performs measurement of an azimuth of an object, there is known, for example, a device using a beam switching system, which arranges a plurality of receiving antennas having a narrow beamwidth, capable of determining an azimuth easily and having a differently oriented beam from each other, and connects those receiving antennas sequentially one by one. There is also known a radar device, for example, using a mechanical system, which swings a single receiving antenna mechanically, thereby to change the direction of a beam. Further, there is known, for example, a radar device using a electronic system, which uses a phased array antenna and changes the direction of a beam electronically. In conventional radar devices, an azimuth of an object is measured utilizing the above mentioned systems to receive a reflected wave from an object.
The case in which a radar device performing azimuth measurement is used, for example, as a radar device installed in a vehicle will be explained hereunder. First, explanation will be given of an example of a radar device using a beam switching system, where the range of azimuth measurement is determined to be, for example, about 10deg!. If an azimuth of an object is to be detected, for example, for each 1deg! of this range of measurement, it is necessary to arrange planar antennas having a beamwidth of 1deg! at positions shifted by each 1deg!, in order to determine an azimuth of an object uniquely. Therefore, ten antennas are needed in all.
Generally, there lies the following relation between directivity and dimension of an antenna: EQU .crclbar.=k(.lambda./D) (1)
where
.crclbar.: half power beamwidth of an antenna PA1 k: half power beamwidth coefficient which is determined depending on a feeding method PA1 .lambda.: used wavelength PA1 D: horizontal array length of an antenna.
As is clear from the above, directivity of an antenna is inversely proportional to the dimension of an antenna, if the same frequency and the same feeding method are used. When a planar array antenna as described above is used and the half power beamwidth coefficient k and used frequency are, for example, 50.8.degree. and 60.5 GH.sub.z ! (wavelength .lambda.=0.004955m!), respectively, the expression (1) is as follows: EQU 1=50.8(0.004955/D) EQU D.div.0.252 m!.
Thus, in this case, ten of planar array antennas of about 25cm! in each side are needed as receiving antennas.
As described above, a radar device using a beam switching system needs to arrange many antennas when a wide range of azimuth measurement is desired. Therefore, a radar device using a beam switching system is not suited to be installed in a vehicle which has only limited space for installation, and the range of application of this kind of radar device is limited. A radar device using any of the other systems mentioned above is not suited to be installed in a vehicle, if it likewise needs to arrange many antennas.
In addition, a radar device using a mechanical system as mentioned above needs a motor, an actuator and their power supply in order to swing an antenna. Thus, a radar device of this kind takes a long time to change an antenna, and takes high manufacturing cost in order to ensure reliability in precision of its mechanical parts. On the other hand, a radar device using an electronic system as mentioned above needs a phase controller for each antenna element, which controls an electric wave so that it may have a phase calculated according to a predetermined operation, and thereby controls directivity. However, a phase controller for a band of micro wave or millimeter wave, which is the band used by a vehicle, is expensive, so that a radar device using an electronic system is not suitable for a vehicle.
As a system for use in a radar device for measuring an azimuth, of which high reliability in precision and reduction of cost are expected, there are known a sequential lobing system and a monopulse system. These are the systems utilizing signals from a plurality of antennas whose beams are different in direction from each other. As shown in FIG. 25, antennas 3, 4 used in these systems are so arranged that patterns of their beams 1, 2 partly overlap with each other, so that values of received strengths relative to the same object by the respective beams can be compared with each other.
For example, when an object (target) 10 exists within an area where the two beams 1, 2 overlap each other, a radar device needs to have received powers of reflected waves from the object which are detected by the respective beams, in order to obtain an azimuth of the object. Received power by each beam is determined depending upon a radar cross section of an object which falls within a detection range of each beam and a beam pattern of each beam. Therefore, conventional devices calculate a ratio of received powers (hereunder referred to as "power ratio"), compares the calculated power ratio with antenna beam patterns, and thereby obtains an azimuth of an object.
When there is a disturbance in an electric wave due to low received power, an influence of noises or so forth, a radar device may fail to detect a reflected wave though an object exists within its range of measurement, so that the object may be missed. Such phenoma will be hereunder referred to as "omissions". In order to deal with this problem, in conventional radar devices, a tracking part is provided to accumulate instantaneous results of measurement for a predetermined number of times, thereby to supplement measured values corresponding to "omissions" produced in measurement.
However, in the sequential lobing system and the monopulse system, measurement of received power is needed. Therefore, radar devices using those systems need to have an automatic gain controller (AGC), an IQ demodulator, an A/D converter, a sampling circuit and so forth, so that they have the disadvantage of large circuit formation.
Further, the monopulse system has a problem that it needs complicated circuit formation as compared with the sequential lobing system, since it performs processing of signals such as RF signals basically in the manner of parallel processing.
Furthermore, both of the above systems have a problem that "omissions" have a large influence on measurement of an azimuth, so that sufficient supplement relative to "omissions" may not be performed.
Furthermore, in a transmit-receive composite directional characteristic of an actual multibeam antenna, a beam pattern shows a rise in gain called "main lobe" which forms a central part of a beam pattern, and a rise in gain called aside lobes which is produced on each side of a main lobe (See three beams L, C, R of a multibeam antenna shown in FIG. 26). A side lobe is a problem since it distorts power ratio of beams, and any of the above mentioned systems needs to consider this problem. In measurement of an azimuth, it is desirable that all the side lobes, i.e., the lobes excepting a main lobe are compressed to a very low level.
In an actual beam pattern, side lobes are necessarily produced and cause such a problem, when a azimuth is to be obtained from a power ratio, that an area capable of azimuth judgment is narrowed due to the influence of side lobes, or that a plurality of azimuths correspond to a single power ratio so that a correct azimuth is difficult to judge.
Further, for example, in conventional systems, an index of reliability in azimuth Judgment is formed based on an absolute value corresponding to a power. A power ratio relative to a given target, based on which an azimuth is obtained, momentarily varies largely, so that smooth processing is needed. For smooth processing, for example, if an average is taken for a plurality of times of power measurement, such may happen that the average obtained in the case in which large power has been only once accidentally measured due to an influence of noise or so agrees with the average obtained in the case in which power close to the average has been measured a plurality of times. The former case is a problem, since it does not provide an appropriate evaluation of power, so that it leads to a false judgment of an azimuth.
Further, conventional systems have such a problem that when an object is at a large distance or when it is small, the area where an azimuth measurement based on a power ratio relies on only one beam is increased, so that resolution in azimuth measurement is lowered.